[Piglit] [PATCH] shaders: Make glsl-fs-lots-of-tex less susceptable to rounding error.

[Ilia Mirkin]

Kinda like http://patchwork.freedesktop.org/patch/17718/ ? :) The
feedback I got (although not apparently in e-mail) was that it was
shady. OTOH, the test doesn't do what it claims to due to the CSE that
now happens, so perhaps it should just get deleted.

On Fri, Jul 11, 2014 at 4:01 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> The only reason this test passed before is because we were rounding wrong
> in texture upload.  Previously, the shader started with 0.01, effectively
> multiplied it by 34, and then compared it to 0.4.  The problem is that,
> when converting from float to unsigned byte, you get 0.01 * 255 = 2.55.
> Previously, we were rounding this up to 3.  Then 3 * 34 = 102 and
> 102 / 255 = 0.4.  However, the OpenGL spec specifies that you should simply
> cast the floating point value to the integer value and this rounds down to
> 2.  The result is that you get 2 * 34 = 68 and 68 / 255 = 2.667 which is
> nowhere close to 0.4.
>
> The new version simply starts with 0.4 and then divides by 4 at the end of
> the shader to get back to 0.4.
>
> Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
> ---
>  tests/shaders/glsl-fs-lots-of-tex.shader_test | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tests/shaders/glsl-fs-lots-of-tex.shader_test b/tests/shaders/glsl-fs-lots-of-tex.shader_test
> index a31778f..9056774 100644
> --- a/tests/shaders/glsl-fs-lots-of-tex.shader_test
> +++ b/tests/shaders/glsl-fs-lots-of-tex.shader_test
> @@ -34,12 +34,13 @@ void main()
>         vec4 p = texture2D(tex, cst.xy) + texture2D(tex, cst.xy);
>         vec4 q = texture2D(tex, cst.xy) + texture2D(tex, cst.xy);
>
> -       gl_FragColor = a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p + q;
> +       vec4 sum = a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p + q;
> +       gl_FragColor = sum / 34.0f;
>  }
>
>  [test]
>  uniform int tex 0
>  uniform vec4 cst 0.0 0.0 0.0 0.0
> -texture checkerboard 0 0 (8, 8) (0.01, 0.0, 0.0, 0.0) (0.01, 0.0, 0.0, 0.0)
> +texture checkerboard 0 0 (8, 8) (0.4, 0.0, 0.0, 0.0) (0.4, 0.0, 0.0, 0.0)
>  draw rect -1 -1 2 2
>  relative probe rgb (0.75, 0.75) (0.4, 0.0, 0.0)
> --
> 2.0.0
>
> _______________________________________________
> Piglit mailing list
> Piglit at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/piglit